Since 1985, a new design of heat exchangers with a new type of twisted tube configuration started to be installed worldwide under various field conditions. The main advantage of these twisted tube heat exchangers is their excellent heat transfer performance and smaller footprint (up to 30% smaller than conventional round tube heat exchanger). However, commonly used nondestructive inspection techniques are not able to perform a reliable and accurate inspection of the twisted tubes.
Currently, twisted tubes found in heat exchangers are being inspected with electromagnetic methods: Remote Field Testing (RFT) is used for all ferromagnetic material such as carbon steel, duplex steel and nickel, while Eddy Current Testing (ECT) is used for non-ferromagnetic material such as titanium, stainless steel, brass and copper-nickel. These two inspection techniques make use of cylindrical probes (featuring a circular cross-section). The largest cylindrical object that can be inserted, pushed and pulled freely in the twisted tubes determines the diameter of the aforementioned probes. Because of their very nature, electromagnetic methods work best when probes are closest to the surface being examined. The circular probes currently in use by the industry are therefore delivering sub-optimal performances: flaw detection, flaw characterization and sizing are often problematic due to poor signal-to-noise ratios.
These RFT and ECT probes are well known in the art but are not ideal to locate, characterize and assess defect size during the non-destructive examination of twisted heat exchanger tubing. Indeed, flaws located on the tube wall furthest away from the center (aka the crest) produce much smaller, distorted, signals than same-size flaws located in the narrow section closest to the tube center (aka the dip). This translates into a reduced probability of detection. The common sizing method for determining flaw depth is inaccurate because of the reduced sensitivity and the large tube signal variations coming from the various flaw locations (crest or dip).
Moreover, because the heat exchanger has a plurality of twisted tubes in close contact to one another, the inspection results are greatly affected by the presence of the adjacent tubes. Therefore, even if a conventional circular probe can detect some flaws in the crest (with a reduced sensitivity) when the tube is taken in isolation, the detection capability is reduced considerably when the same flaw is located in a tube bundle, under real-life conditions. This reduction in sensitivity is particularly important when using the RFT technique.